1. Technical Field
The present invention relates to a method for forming a transistor of a semiconductor device.
2. Description of the Related Art
As semiconductor devices serve a broader range of applications, there are urgent needs associated with development of processing facilities or processing techniques for manufacturing semiconductor devices that are low in manufacturing costs yet superior in integrity and electrical properties. In relation to this, many started diversified researches on size reduction of elements, such as, reduction of thickness of a gate dielectric film and reduction of junction depth of source/drain have been researched.
However, since a semiconductor device must meet the strict design rule of 10 μm or less, in addition to the size of transistors including the aforementioned elements, length of channels of their peripheral circuits should be decreased. This makes it very difficult to manufacture transistors with stable performance.
For example, as the depletion layer of a source/drain region penetrates into a channel and reduces an effective channel length, the short channel effect has occurred. As a result, it is difficult to secure a sufficient data retention time in an element. Moreover, a junction leakage current increases because of a reduced process margin.
In order to increase the effective channel length in a conventional method, an attempt has been made to form a recessed channel gate transistor by etching an active region of a semiconductor substrate.
However, as shown in FIG. 1, in case of forming a trench for the recessed channel gate by etching the active region of a silicon substrate 1 having an isolation oxide film 3, isolation oxide film 3 can be etched deep inside during a cleansing process using phosphoric acid. As a result, an unnecessary trench having a large width is formed. Because of this, in a subsequent word line forming process, a word line pattern 5 having a vertical profile 9 is formed on the active region of semiconductor substrate 1, while a word line pattern 5-1 of a sloped sidewall 11 is formed on isolation oxide film 3 having a large width (a).
Accordingly, a bridge is formed between a storage node (or a bit-line node plug) and the word line. This brings a defect into operations of elements and further decreases productivity and yield of semiconductor devices.